1. Technical Field of the Invention
The present invention relates to a semiconductor laser unit, and more particularly, to a semiconductor laser unit constituting an optical pickup for writing/reading information into/from a recording medium such as an optical disk, in particular, including a digital versatile disk (DVD) and a compact disk (CD), for example, and an optical pickup device provided with such a semiconductor laser unit.
2. Prior Art
In recent years, as recording media for not only music information but also video information, CDs (CD-ROM, CD-R, CD-RW, etc.) and DVDs (DVD-ROM, DVD-RW, DVD-RAM, etc.) have rapidly widespread. At the same time, optical disk drives for writing information into such recording media and reading written information from such recording media have also rapidly widespread. Optical pickup devices, which serve as the key parts of such optical disk drives, have been strongly requested to attain higher power for supporting high-speed recording, enhance functions for supporting both CD and DVD standards and reduce the size with thinning of optical disk drives. Accordingly, semiconductor laser units used for such optical pickup devices are requested to improve the heat dissipation of packages for attaining higher power, support a larger number of pins for enhancing functions and provide a narrow package structure for reducing the size.
A semiconductor laser unit for a conventional optical pickup device will be described, taking as an example a semiconductor laser unit described in Japanese Patent Gazette No. 3412609, with reference to FIGS. 6A and 6B.
FIG. 6A is a top view of a conventional semiconductor laser unit, and FIG. 6B is a cross-sectional view of the conventional semiconductor laser unit taken along line VIb-VIb in FIG. 6A.
As shown in FIGS. 6A and 6B, the conventional semiconductor laser unit includes: a lead frame 1600; a silicon substrate 1630 mounted on the lead frame 1600; a semiconductor laser 1640 placed on the silicon substrate 1630 in the center of a package 1610 for outputting laser light upwardly; light-receiving elements 1620 as chips placed on the silicon substrate 1630; the package 1610 molded with a resin mold for sealing the lead frame 1600; a 45-degree reflector for reflecting the laser light toward the top of the package 1610; and a hologram element 1650 with a grating pattern 1650b formed on the bottom surface and a hologram pattern 1650a formed on the top surface. A circuit for processing reflected light from an optical disk received by the light-receiving elements 1620 is also formed on the silicon substrate 1630.
In the conventional semiconductor laser unit described above, as shown in FIG. 6B, output light 1660 from the semiconductor laser 1640 is reflected with the reflector toward the top of the package 1610, is diffracted with the grating pattern 1650b and then passes through the hologram element 1650. The output light 1660 then travels through optical components (not shown) such as a collimating lens and an objective lens to reach an optical disk (not shown). Reflected light 1670 from the optical disk travels in reverse the same route as the output light 1660, is diffracted with the hologram pattern 1650a and is then incident on the signal processing circuit and the light-receiving elements 1620.
When it is attempted to achieve higher power, function enhancement and size reduction for an optical pickup device with the semiconductor laser unit having the construction described above, two major problems arise: one is improvement in heat dissipation associated with achievement of higher power, and the other is narrowing of the pin pitch associated with achievement of function enhancement and size reduction.
In general, in an optical disk drive adapted to high-speed recording, it is required to output high-power light of 200 mW or more from the semiconductor laser unit. To accomplish this, the temperature of the semiconductor laser 1640 will rise due to increase of the drive current of the semiconductor laser 1640, and this will degrade the reliability of the semiconductor laser 1640. To ensure stable driving of the semiconductor laser 1640, heat generated during driving must be dissipated efficiently. However, in the conventional semiconductor laser unit described above, the package 1610, which is formed of a resin having low heat conductivity (about 0.5 W/m/deg), is highly heat resistive in structure and thus fails to dissipate heat efficiently.
Also, when it is attempted to reduce the size of the package 1610 in the conventional semiconductor laser unit having the construction described above, the width of the package 1610 will be narrowed, and this will result in imposing a limitation on increase of the number of pins that may become necessary with function enhancement. The reason is that while the pin pitch must be narrowed to increase the number of pins with the narrowed width of the package 1610, a pitch of about 0.4 mm is the minimum possible pitch, and no pitch narrower than about 0.4 mm is available, in the current processing technology for the lead frame 1600.
A semiconductor laser unit with improved heat dissipation is described in Japanese Laid-Open Patent Publication No. 2003-67959. In this semiconductor laser unit, a metal substrate is attached to the bottom of a laser unit portion on which a semiconductor laser is mounted, to enable efficient dissipation of heat generated in the semiconductor laser.
In an optical head device described in Japanese Laid-Open Patent Publication No. 8-227532, an optical element structure is attached to a plate made of metal, to improve the heat dissipation efficiency.
A semiconductor laser unit described in Japanese Laid-Open Patent Publication No. 2002-198605 uses a bent flexible substrate as a wiring substrate, and this can reduce the wiring width. Moreover, this conventional semiconductor laser unit can dissipate heat generated in a semiconductor laser efficiently via the backside of a metal island.
However, the conventional devices described above have the respective problems as follows. The semiconductor laser unit described in Japanese Laid-Open Patent Publication No. 2003-67959 finds difficulty in achieving both size reduction and function enhancement.
The optical head device described in Japanese Laid-Open Patent Publication No. 8-227532 finds difficulty in thinning the device because a flexible substrate protrudes from the plate. Moreover, the optical head device may possibly be contaminated with dust generated during bonding of a case.
The fabrication process for the semiconductor laser unit described in Japanese Laid-Open Patent Publication No. 2002-198605 is complicated and finds difficulty in shortening the work time, and this makes it difficult to secure the placement accuracy of each component sufficiently. Moreover, in this conventional semiconductor laser unit, the flexible substrate is bonded to the metal island with terminal portions of the flexible substrate being bent. This complicates the work during fabrication, and thus may possibly fail to obtain sufficient bonding strength at connection portions.